From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the medical diagnostic field to reduce the quantity of spent solution. Needed in this regard is a technology relating to thermographic photosensitive materials for use in the medical diagnostic and photographic fields which can be effectively exposed by means of laser image setters or laser imagers and produce clear black images of high resolution and sharpness. These thermographic photosensitive materials eliminate a need for wet processing chemicals and offer a simple, environmentally friendly, thermographic system to the customer.
In photothermographic elements comprising a non-photosensitive organic silver salt, a photosensitive silver halide, and a reducing agent, the non-photosensitive silver salt serves as a silver ion supplier during development. It was often believed that a greater number of silver halide grains presenting development starting sites leads to an increase of covering power and an improvement in graininess. However, with respect to the covering power, since it is considered that the size of developed silver is determined by the mass of silver which is, in turn, determined by the radius of silver ion supply during heat development, known as the sphere of influence, simply increasing the number of grains leads to more hardening of a characteristic curve, failing to achieve a sufficient increase of covering pouncer.
Also an increased number of grains can aggravate graininess. Excessive coating of silver halide grains makes overt the problem of silver tone aggravation. For this reason and others, it is very difficult to achieve a covering power in excess of 2.5. It is also very difficult to exceed the graininess of conventional silver halide photosensitive materials subject to wet development.